The acceleration rate, full speed velocity, and deceleration rate of a hydraulically driven elevator car, regardless of whether the car is mounted on the end of a plunger of a hydraulic cylinder, or rope driven in response to plunger movement, are all subject to deviation from desired or reference values. The deviation is responsive to the load in the elevator cab, the car travel direction, the location of the car in a building, and the temperature of the hydraulic oil. Deceleration of a hydraulic elevator car is initiated by de-energizing a high speed valve, and the car then decelerates until it reaches a predetermined landing speed. Then the car travels at landing speed until it enters a leveling zone for the target floor, defined by indicia in the hatchway. The car is stopped by de-energizing a stop valve. The position of the car in the building, as affected by buoyancy of the plunger, the oil viscosity, which changes with temperature, the car loading, and the car travel direction, all have a bearing upon when the car will reach landing speed. If the car reaches landing speed too soon, it will creep to the landing or leveling zone, deleteriously affecting elevator service, as well as increasing the anxiety of passengers who sense the amount of time between a change in deceleration rate when landing speed is reached and the opening of the car doors. Electrical power is also wasted when a hydraulically driven elevator car is creeping into a floor in the up travel direction, as the pump motor is operated in the up travel direction.
Many different arrangements are utilized in the prior art for overcoming these disadvantages, but all require one or more hardware items which add cost to the installation, such as special valves which are compensated for load and/or oil temperature and/or travel direction, special detectors, which detect car speed, car load, oil temperature, and the like.
It would be desirable, and it is the object of the present invention to improve the landing of a hydraulically driven elevator car, without adding any additional hardware, using hardware and signals which are already present in any computer operated hydraulically driven elevator car.